Diamond abrasive



July 19, 1949. R. F. CLlNE DIAMOND ABRASIVE Filed Nov. 6, 1945 25 I I iROBE/er E Q WE Patented July 19, 1949 assignor to Norton a corporationof Company, Worcester, Mass.,

Application November 6, 1945, Serial No. 620,982

1 Claim.

3 The invention relates to diamond abrasives. One object of theinvention is to provide a diamond grinding wheel bonded with a hard,heat-resistant bond. Another object of the invention is to produce asintered metal type bond of great hardness. Another object of theinvention is to provide a method for the manufacture of diamondabrasives yielding a hard metallic bond under such conditions that thediamonds are neither oxidized nor graphitized nor otherwise impaired.

Another object of the invention is to provide for diamonds a bond of themetallic class which is harder acting than the non-ferrous metal bondsand also harder than most sintered steel bonds now known. Another objectof the invention is to provide a diamond abrasive useful in new types ofgrinding operations. Another object of the invention is to provide agrinding wheel having one or more of the above characteristics and whichis nevertheless free-cutting.

Another object of the invention is to produce a diamond grinding wheelespecially for cutting or grinding of hard, brittle substances. Anotherobject of the invention is to provide a hard and somewhat friable bondwhich will allow dull diamond grains readily to break away from thewheel to permit new sharp edges of diamond to be presented to theworking face. ject of the invention is to provide a whole range of bondsvarying in hardness from about Rockwell A 85 or higher down to thehardness of bronze and varying from quite friable or brittle to fairlyductile.

Another object of the invention is to provide a well bonded diamondgrinding wheel. Another object of the invention is to provide acombination of elements which, in powder form, will quickly sinter toproduce a uniform composition to bond diamond abrasive grains. Otherobjects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the features of construction,combinations of elements, and in the several steps and relation andorder of each of said steps to one or more of the others thereof, all aswill be illustratively described herein, and the scope of theapplication of which will be indicated in the following claim.

In the accomp nyi drawings illustrating two Another obof many possibleembodiments of the mechanical features of this invention,

Figure 1 is an axial sectional view of a graphic mold assembly andinduction heating apparatus;

Figure 2 is an end view of a diamond wheel made in the mold of Figure 1;

Figure 3 is an axial sectional view taken on the line 3-3 of Figure 2;

Figure 4 is an axial sectional view of another graphite mold assemblyfor the production of a hollow cylindrical wheel including anon-abrasive portion;

Figure 5 is an end view of the wheel made in the mold of Figure 4; and

Figure 6 is an axial sectional view, taken on the line $6 of Figure 5.

For the grinding of difierent materials and under difierent conditionsand to produce specific results, many different kinds of abrasives andbonds have been used. Diamond abrasive bonded with synthetic resinfulfills particular needs while vitrified bonded diamond wheels arepreferred for certain operations. Where hard bonded diamond wheels havebeen wanted, resort has usually been had to metal bonds. Sinteredcopper-tin bond has been used with great success to bond diamondsmaking, relatively speaking, a hard bonded diamond wheel. Sintered steelhas been used to some extent to provide a still harder acting bond fordiamonds. I have discovered a composition and a method for theproduction of diamond abrasives with a still harder bond which icalcomposition:

Per cent B. 17.32 C 21.62 F 0.03 Undetermined L03 Percen e MIMI-l8 by wtgg 1 Finer than 1 I provide a quantity of iron powder. The iron powderthat was actually used would pass through a 325 mesh screen and analysisindicated it to be more than 98% iron. It was obtained fromacommercialsource on the open market.

I made a mixture of these powders to give a volume percentage of 23 B40to 7'7 iron. Actually, to provide this percentage ratio, I took 17.8grams of B40 and 182.2 grams of iron. The two powders were mixedtogether in a standard type of pan mill for a period of about 18 hoursto insure uniform mixing. A weight of 15.7 grams of this dry mixture wasthen taken as bond for the wheel. To this was added 2.77 grams ofdiamond grainof 100 grit size to produce a resulting wheel containing 25volume per cent of diamond. This small amount of mixture which totalled18.47 grams was mixed with a spatula by hand to reasonable uniformityand then loaded into a graphite mold.

Referring now to Figure 1, the mold consisted of an outer hollowcylinder I of graphite with a bore fitted with hollow cylindricalgraphite plungers 2 and 3 which would just slide in the bore andthemselves having a 3/4" bore in which a cylindrical graphite core rod 4fitted. The wheel mixture was placed in the annular zone 5 between thecore rod 4 and the outer cylinder I. The plungers 2 and 3 moved fromboth ends to compress the mixture in the annular zone 5. The mixture,which was dry. was-compacted in the mold by a moderate cold pressingoperation under a. pressure slightly less than 1000 pounds tothe squareinch. I

The mold was then placed between graphite rods 6 and I and inside of ahigh frequency induction furnace coil 8 havinga refractory lining. Whileheating the mold it was subjected to a pressure of 2260 pounds-persquare inch by a press ram l0 acting against therod 6, the rod 1 beingsupported by the press platen II. The

- heatingand' pressing were continued until the required contractionshown by a press gage had been obtained toyield the desired length ofwheel. The moldassembly was then removed from the furnace and allowed tocool slowly in diatomaceous earth powder. 1'

It is usually desirable to make a diamond grinding wheel integral with aback or center of nonabrasive properties so that all orsubstantially allof the diamond material may-be used in grinding, leaving little or nonein the discarded stub. Figure 4 illustratesa mold. to make such a wheel.An outer cylinder ll of graphite is fitfed with hollow cylindricalgraphite plungers i8 and I4 which will just slide inthe bore of thecylinder l2 and in the bores or the plungers l3 and H is a hollow coreIt. also of graphite. The

annular zone ii of this mold is partly filled with a mixture such asalready described, and partly with such mixture without the diamonds, i.e. with iron and boron carbide. The two mixtures are cold pressed in themold as already described and then the mold is placed between pressureelements and in a high frequency induction furnace of a size to receiveit and the mixtures are compacted under heat and pressure of the orderof and as already described. A feature of the mold of Figure 4 isthat-the core I5 is hollow and thinwalled so that the piece can c001from the inside as well as from the outside to prevent fracture of theproduct due to cooling stresses. This is important for relatively largediameter wheels.

The grinding wheel 20 made in the mold of Figure l is shown in Figures 2and 3. When the cold mold was broken apart and the wheel 20 cleaned ofgraphite, it was mounted on a. mandrel and found to rotate withremarkable truth. While rotating it was dressed with fine boron carbidegrain of about 200 grit size to cause the diamond grains to projectabove the surface of the bonding material.

This wheel 20 had a good distribution of diamond grains around itsperiphery and across its ends. The diamonds themselves were undamaged bythe heat treatment due to the short time during which they were above1000 C. (about five minutes). The actual temperature on the outside ofthe mold was 1085 C. The diamonds were well stuck orattached in thewheel, indicating that good'bonding had been achieved. I

Grinding tests were made with the wheel 20 in comparison with 'astandard metal bonded diamond wheel containing the same concentration ofdiamonds, the bond being 18.5% tin and 81.5% copper (by weight), andalso in comparison with a standard resinoid (phenolic resin) bondeddiamond wheel, also having the same concentration of diamonds. Thegrinding machine was a Heald 72a internal grinder, wheel speed 25,000 R.P. M. The following tables give the grinding results for particularmaterials.-

Material on Diameter 7 Mill Tenn:

5 Diameter Of the invention Btandard'Metal bonded when Wear on DiameterMils Wm] Diameter Mils 0f the invention Standard Besinoid Bonded Removedon MaterlalR-e movedo'n Tents III Grinding Sondersons cartridge diesteel Material Re- Wheel Wear Wheel on Diameter gm Mill Mile

! the invention 0.1 12 0 Standard Metal Bonded. 0. 7 12. 0

example of a relatively thin zone of abrasivebearing bond moldedintegrally with another zone having the same alloy composition but noabrasive. A wheel such as the wheel 2| or any other integral shape canbe mounted on a steel shank or any other suitable mounting. It can bebrazed to a shank by a subsequent heating operation and the bond willnot be deleteriously affected as long as the brazing temperature is lessthan 100 0.

The above chemical analysis should not be considered as a limiting casesince, in the commercial preparation of boron carbide, the boron con-,tent may, if desired, be raised to 95% with the balance mainly carbon.The presence of the carbon was considered to be of doubtful value. Since17.8 grams of boron carbide plus 182.2 grams of iron ads up to 200 gramsof mixture, the proportion is 8.9% of boron carbide to 91.1% of iron byweight. Since there was but 21.62% of carbon in the boron carbide, therewas only 1.94% of carbon in the mixture.

In an attempt to find out the true nature of the'bondingmaterial of theinvention, I prepared a sample of 25% by volume of boron carbide of theforegoing analysis and 75% by volume of Fe (this differs only slightlyfrom the proportions used in making the wheels described) This mixturewas heated under pressure as already set forth and the compacted samplewas then powdered and subjected to X-ray analysis. Then another mixturewas made of 40% B40 and 60% Fe by volume, likewise heated underpressure. powdered and subjected to X-ray analysis. The first of thesewill be called sample No. 1 and the second will be called sample No. 2.Excellent powder photographs were obtained from both samples. Thefollowing conclusions are believed to be correct:

(a) The principal phase present in both samples No. 1 and No. 2 wasFeaB, sometimes referred to as F6432, which contained little or nothingin solid solution.

(0) Sample No. 1 contained little or no B40 or Fe.

(0) Sample No. 2 seemed to contain a small amount of each of B40 and Fe.

(:2) No indication of FeB or of 13 could be found in either of sampleNo. 1 or sample No. 2.

(e) Graphite was indicated by the X-ray photographs to a slight extentin sample No, 2. From all considerations graphite should be present inboth of the samples and to a greater extent in sample No. 2 than insampleNo. 1.

(I) From all considerations there is doubt as to the presence of FeaC ineither sample. The X-ray photographs failed to indicate any.Furthermore, the sintering time was too short to make the formation ofFeaC likely.

It is, therefore, considered that the principal sintering reaction maybe indicated by the for- This would indicate that to combine all of theiron and boron and provided the boron carbide is represented by theformula B40, 11% B40 to 89% Fe by weight should beused. But, asindicated, an excess of boron over that required by the formula 1340 maybe present (in solid solution) in the boron carbide powders. In suchcases more iron may be provided to combine all of the boron to FezB.

In order that the various weight and volume percentages alreadymentioned may be seen at a glance, the following table is provided:

TABLE IV Volume per cent B10 Weight per cent 10.0 3.87 23.0 Formula from8.9 which were prepared wheels of the two examples given 25.0 Sample No.l 9.67 28.4 Rgti) fol 8F 11.0 4F B o l e -r e: 30.0 11.6 40.0 Samplg No.2 15.5

c 90.0 06.13 77.0 Formula from 91.1 which were prepared wheels oi thetwo examples given 75.0 Sample No. 1 90.3 71.6 Ratio ior 89.0

B 8Fe 4F02B C 70.0 88.4 60.0 Sample No. 2 84.0

The composition of sample No. 1 appears to yield the highest hardnessvalues, indicated by tests to be in the range of Rockwell A scale 80 to85. This is very hard. Rockwell A 87 is about 600 times as hard as plateglass in wear resist-. ance. Commercial cemented tungsten carbide has a.hardness range on the Rockwell A scale of around 86 to 91. It may bedesirable in some cases to provide a bond which, while much harder thanthe widely used copper-tin bonds, isyet not so hard as the above. Ingeneral, thehardness and brittleness of the bond can be reduced bydecreasing the proportin of boron carbide,

For example, with 10% by volume of B40 and 90% by volume of Fe (seeTable IV the resulting metallic material becomes more ductile andactually can be deformed without snap fracturing; This is doubtless dueto a. substantial proportion of iron as ferrltein the composition.Assuming all of the boron combined with iron to form FezB, thiscomposition is by weight:

Per cent Free iron 71.86 Combined iron 24.27 Combined boron-l-carbon3.87

Total 100.00

While boron carbide, B40, is of great hardness,

and is in fact considered next to the diamond itself in hardness, it hasbeen noted that 90.3% iron, the remainder 1840, produced the hardestbond. This is doubtless because the particular time-temperature factorfor sintering does not cause the boron carbide grains readfly to bond.shows, upon metaliographic inspection, discrete crystals of 1346. Thesetend to weaken the structure somewhat. Yet for certain grinding resultsit 'is desirable to have a friable bond. 80, therefore, I may use asmuch as 40% 1340 by volme (see Table IV) or, stated in another way,

- as little as 84.58% by weight of iron (sample No.

2). The low limit of iron is, therefore, stated,

in an even percentage, as 84% by weight.

Instead of sintering the mixture of boron carhide and iron undercombined heat and pressure,

the grinding wheel or other abrasive body may first be cold pressedunder a relatively high pressure and subsequently sintered withoutpressure. To substantiate this, a test bar was made by pressing amixture of 23 volume per cent of 34C and 77 volume percent of iron undera pressure of 10,000 pounds per square inch without any heat. later thistest bar was heated in an induction furnace in a vacuum for fourhours ata temperature of 1000 C. This process made a strongbar and accordinglyis deemed to be useful for the manufacture of grinding wheels containingdiamonds. From certain tests made by my associates it is clearthat thediamonds will not be detrimentally graphitized by such heat treatmentinthe presence of the range of compositions according to the presentinvention, provided carbon ispresent even in combined form to the extentof at least 3% by weight of the bond, as will usually be the case. Ineither method of preparing the product the bond is a silvery metallicproduct having a density of from 6.0 to 7.0. Accordingly if formanufacturing reasons it is desired to do the heating in a furnace whichcannot be associated with pressure apparatus, cold pressing andsubsequent sintering may be re- Y sorted to.

While the utility of the invention is now known in connection withdiamond abrasives, it is contemplated that the bonding ingredientsmay beused to bond other hard materials such as aluminum oxide, and especiallysilicon carbide, not

' material and I wish to claim the same separately.

It will thus be seen that there has been provided .by this invention anarticle and a method in which the various objects hereinabove set forthtogether with many thoroughly practical advantages are successfullyachieved. As various possible embodiments might be made of themechanical features the above invention and as the art herein describedmight be varied in various parts, all without departing from the scopeof the invention, it is to be understood that all matter hereinbeforeset forth or shown in the accompanying drawings is to be interpreted asillustrative and not in a limiting sense.

I claim:

An abrasive composition comprising diamond grains bonded with a sinteredmixture containing from 25% to Fe'zB by weight and from 91% to 97.8%iron by weight, including both free and combined iron.

ROBERT F. CLINE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 952,290 Whitney Mar. 15, 19101,562,043 Pacz Nov. 17, 1925 2,046,912 Kormann et al.' July 7, 19362,076,952 Krathy Apr. 13, 1937 2,358,459 Kelleher Sept. 19, 1944 FOREIGNPATENTS Number Country Date 491,659 Great Britain Sept. 6, 1938 OTHERREFERENCES Mellor, Comprehensive Treatise on Inorganic and TheoreticalChemistry, vol. 5.

